The present applicant has developed, prior to the present invention, a process for preparing a fluorine-containing elastic copolymer having remarkably improved vulcanization reactivity and compression set of the vulcanized rubber and a vulcanizable composition made from the copolymer. Namely, the process for preparing the fluorine-containing elastic copolymer is a process for preparing a copolymer of vinylidene fluoride containing 28 to 92% by mole of vinylidene fluoride unit and at least one other monomer comprising (a) polymerizing vinylidene fluoride and at least one other monomer by the use of a water-soluble radical polymerization initiator in the first stage to produce a copolymer and (b) then polymerizing in the second stage of the polymerization by the use of an oil-soluble radical polymerization initiator in the presence of the copolymer prepared in the first stage to produce a copolymer, and the vulcanizable composition of the fluorine-containing elastic copolymer is a composition prepared from the copolymer (JP-A-62391/1977 and JP-A-76359/1977). However, in all of their experimental examples thereof, an amount of the water-soluble polymerization initiators is as large as about 0.08 to 0.6% by weight, and the obtained fluorine-containing elastic copolymer is not enough for recent strict requirements with respect to improvement of processability of fluorine-containing rubbers.
One of particular requirements regarding the fluorine-containing rubbers is improvement of molding processability thereof. Since the fluorine-containing rubber is high in material cost as compared with other rubber materials, when the vulcanization speed is fast and its flowability is good, such a rubber makes it possible to produce a molded article of the fluorine-containing rubber having complicated shape effectively and to lower a total production cost. Particularly, in order to conduct injection molding which is more effective as compared with compression molding, it is required to exhibit more excellent flowability, rapid vulcanization and effective crosslinking, namely it is required that irrespective of a low viscous polymer, vulcanization can be conducted in a short period of time and yet sufficient physical properties at normal state are exhibited. On the other hand, the vulcanization accelerator comprising a quaternary phosphonium salt which is often used when the polyhydroxy aromatic compound is used as the vulcanizing agent provides a molded article exhibiting excellent compression set and large elongation at breakage. However, there are disadvantages that fine voids appear by foaming particularly when molding at a high temperature for a short period of time, which results in high percentage of defective molded articles and further that there is a possibility of causing contamination of a die due to occurrence of unvulcanized parts because of non-uniform vulcanization. Thus, there was a case where the use of such a vulcanization accelerator was restricted. The present invention provides the process for preparing a fluorine-containing elastic copolymer at a cost nearly the same as before, the fluorine-containing elastic copolymer which is prepared by the process, has highly balanced good flowability, rapid vulcanization and necessary and enough physical properties and also has molding stability which has not been obtained in conventional fluorine-containing elastic copolymers, and a vulcanizable composition which is prepared from the copolymer.